Ionization Region:
As voltage is increased in the ionization region (Region II), there is no appreciable increase in the pulse height. Field strength is more than adequate to ensure collection of all ions produced; therefore, it is not enough to cause some increase in ion pairs due to gas amplification. This region is known as the ionization chamber region.
Proportional Region:
As voltage increases to the proportional region (Region III), the pulse height increases smoothly. The voltage is enough to generate a large potential gradient near the anode, and it imparts an extremely high velocity to the electrons produced through ionization of the gas by charged radiation particles. A velocity of these electrons is enough to cause ionization of other atoms or molecules in the gas. This multiplication of electrons is known as gas amplification and is referred to as Townsend avalanche. The gas amplification factor (A) varies from 103 to 104. This region is known as the proportional region because the gas amplification factor (A) is proportional to applied voltage.